GRAPTOLITES AS INDICATORS OF MAXIMUM FLOODING SURFACES IN MONOTONOUS DEEP-WATER SHELF SUCCESSIONS

Abstract

The graptolitic Early Ordovician succession of the Mount Hunneberg locality, southern Sweden, shows the response of graptolite faunas to sea-level changes. The exposed interval consists of intercalated carbonates and shales at the base, grading into pure black shales in its central and upper part. This facies trend records a deepening of the depositional environment due to an overall sea-level rise. The Mount Hunneberg graptolite fauna is dominated by nearshore, shallow-marine forms found in most layers throughout the succession. Deeper-water pandemic species occur only rarely together with shallow-water graptolites but are dominant at four distinct traceable levels at Mount Hunneberg. This change in graptolite faunal composition is interpreted to indicate sea-level fluctuations. With rising sea level, shallow-water endemics and pandemics migrated landward, and deeper-water pandemic graptolites became increasingly frequent on the shelf. During the peak of the transgression, deep-water forms dominated. With falling sea level, shallow-water forms again appeared and replaced the deep-water graptolites. The four levels with deep-water graptolites at Mount Hunneberg are, therefore, interpreted to represent maximum flooding surfaces. This study demonstrates that changes in the faunal composition of such planktic organisms as graptolites provide a promising tool for recognizing maximum flooding surfaces. As these faunal turnovers are also detectable in monotonous parts of the Mount Hunneberg succession, this biofacies-based approach enables the recognition of maximum flooding surfaces even when no lithologic changes are present, enhancing the applicability of sequence stratigraphic interpretations to monotonous outer shelf strata.